The United States Environmental Protection Agency (EPA) has promulgated rules governing wastewater or sewage sludge quality required for the treated sludge to be land filled or land applied as soil additions or as fertilizer to food crops.
Under 40 CFR 503, there are two sets of sludge pathogen reduction requirements, Class A and Class B. Class A sludge requirements are the most stringent in terms of the pathogen levels required by the EPA. To qualify as Class A, the sludge end product must achieve certain microbial content that is demonstrated by six alternative requirements. Basically, the fecal coliform density in the treated sludge must be less than 1 thousand most probable number per gram total solids sludge on a dry weight basis or the density of Salmonella, sp. bacteria in the treated sludge must be less than 3 most probable number per 4 grams of total solids sludge on a dry weight basis. In addition, certain sludge temperature, pH, and subcomponent density levels must be met under alternative requirements.
Class B sludge requirements are less stringent, where the sludge end product must achieve certain microbial content that is demonstrated by three alternative requirements. Here, fecal coliform density in the treated sludge must be less than 2 million most probable number per gram total solids sludge on a dry weight basis or less than 2 million colony forming units per gram total solids sludge on a dry weight basis.
Several processes for treating sludge have been developed. In U.S. Pat. Nos. 4,781,842 and 4,902,431, there are disclosed processes wherein wastewater sludge containing odor, animal viruses, pathogenic bacteria, and parasites is treated to provide a fertilizer for agricultural lands which can be applied directly to the lands. The treatment process consists essentially of the following steps: mixing the sludge with at least one alkaline material sufficient to raise the pH of the mixture to 12 and above for at least one day, and drying the mixture to produce a granular material sufficient to reduce significantly offensive odor of the sludge to a level that is tolerable, to reduce the animal viruses, pathogenic bacteria, and parasites in the sludge to EPA required levels, to reduce vector attraction to said sludge, and to prevent significant regrowth of the pathogenic microorganisms.
Typically, the alkaline material mixed with the sludge comprises such materials as lime, cement kiln dust, or lime kiln dust. In U.S. Pat. No. 5,275,733, the usable alkaline materials were expanded to include gypsum, fluidized bed ash, lime injected multistage burner ash, dry sulfur scrubbing residue, slag fines, pulverized calcium carbonate, Class C or Class F fly ash, or a combination thereof.
Some processes have utilized the concept of raising the pH in combination with high heat, e.g., greater than 70° C., to nearly sterilize as contrasted to pasteurizing the sludge, thereby killing nearly all of the bacteria both undesirable and desirable. With these “add-on” processes, usually the principal surviving microorganisms are bacterial spores. However, such microbially restricted sludges lose the significant fertility value associated with bioactivity.
When alkaline materials are added to municipal sludges in sufficient mass to raise the pH to at least 11 and to over 12, toxic stresses occur that may affect the disposition of the treated sludge material (EPA 600/2-78-171). For example, the high pH itself may preclude the product use in certain agricultural settings; in addition, the high pH triggers the release of volatile ammonia which itself is toxic and, of course, represents the loss of valuable nitrogen from the potential agricultural product.
The processes of U.S. Pat. Nos. 4,781,842 and 4,902,431 require a drying period which may be effected by, e.g., a windrowing process and results in a product that is above pH 12 and, if produced from an anaerobically digested sludge, emits significant amounts of ammonia. The processes substantially reduce the emission of ammonia by aeration (such as windrowing), but to do so the processes take 3 to 10 days to prepare the product for storage or market. Other alkaline treatment processes, such as in U.S. Pat. No. 5,013,458, using high amounts of CaO and heat above 70° C. for 30 minutes, have been approved by the EPA and have been utilized as the basis for constructing a formula for predicting pasteurization based upon time of incubation at a certain temperature. Such formula is published as part of 40 CFR 503.
In U.S. Pat. Nos. 4,781,842 and 4,902,431, Nicholson and Burnham teach the significant advantages of adding accelerated drying by aeration to alkaline treated sludges to achieve odor reduction and control. When windrows are used, this Nicholson and Burnham process commonly takes between 3 and 10 days to effect the aeration/drying.
U.S. Pat. Nos. 5,275,733 and 5,417,861 provide a faster method of accomplishing the aeration/drying. These patents recognize that an ecologically active population of microflora is critically significant to long-term sludge stability because of its ability: a) to enhance by its own metabolism the carbonation of any residual hydroxides or likewise the catabolism of unstable organics; b) to reduce sludge odors and produce a soil-like odor; and c) to prevent the regrowth of pathogenic microorganisms. These processes thereby avoid excess killing of the microflora.
In addition to wastewater and sewage sludge, another type of sludge called bioorganic sludge presently causes a variety of problems to society with regard to proper disposal or use. These sludges include organic sludges comprised of a material or materials such as, e.g., sludges resulting from production of antimicrobials and other pharmaceutical products, bacterial fermentation sludges, sludges resulting from production of beer and wine, mushroom compost waste, paper mill sludges, sludges that contain microorganisms that have resulted from recycled organic products such as paper products; sludges resulting from the growth of microorganisms for the production of chemicals and organics, industrial sludges and byproducts resulting from the production of microbial products and foodstuffs, and sludges resulting from the animal slaughter industry—particularly if these are digested or otherwise broken down by microorganisms.
Another problematic type of sludge includes organic sludge which is derived from industrial products and byproducts that are comprised in the majority by microbially degradable organic materials not of biological or microbiological origin. These organic sludges might include recycled organic products such as recycled paper and paper products.
The treatment of bioorganic and organic sludges is needed for two reasons. First, bioorganic sludges usually provide an excellent substrate for anaerobic bacterial metabolism, resulting in the creation of noxious odors and community problems. Second, bioorganic and organic sludges without stabilization create runoff problems with non-point source discharge pollution. Accordingly, stabilization may delay entry of nitrogen from the sludges into the ground water both avoiding contamination and allowing longer access for crops to the nitrogen in a stabilized sludge product.
In U.S. Pat. No. 5,853,590, a process is disclosed that performs an alkaline stabilization process for treating bioorganic and organic sludges, in addition to wastewater and sewage sludges. The sludges as biosolid cakes (13–35% solids) are mixed with a combination of lime and mineral by-products (such as lime kiln dust, cement kiln dust, and various coal combustion by-products such as fly ash or flue gas desulfurization by-products) to raise pH to 12 or higher, raise initial solids in the range of about 40%, and generate heat by the exothermic reaction of free lime (CaO) with water in the biosolids. The initial mixture is then introduced into a direct rotary drum dryer and dried to about 65% solids. After discharge from the dryer, the dried material is held in a heat pulse bin for 12 hours at a temperature of 52° C. or higher. The pH of the material is then tested after 72 hours to determine that it is pH 12 or higher.
Concerns with this process include the cost of the alkaline and mineral by-product materials, less material reduction by drying compared to straight biosolids drying, and inflexibility in product characteristics.
Accordingly, there is a need in the art for a method and system for treating and stabilizing bioorganic, organic, wastewater, and/or sewage sludges in such a way as to solve the problems of previous treatment methods, thereby providing a beneficiated soil or fertilizer from the treated sludge product.